This invention relates to an antisubmarine warfare system, particularly to a sonobuoy system for detecting the presence of a discrete source of acoustic vibrations and for determining the direction of said source.
It is known that submarines produce acoustic vibrations which can be detected by means of submerged hydrophones over very long distances, e.g. tens of nautical miles due to the relatively high sound conductivity of water. Thus, detection and classification of submarines can be achieved by a series of sonobuoys spaced 20 to 30 nautical miles apart which transmit the information to an investigating ship or aircraft. Thereafter, the position of the submarine must be determined.
In a known submarine localization system omnidirectional sonobuoys are used which are air or ship-launched in pairs, and the information produced by each sonobuoy of any pair or so called "plant" is telemetered to the investigating ship or aircraft. This information is processed, and the time or phase difference between the time of arrival of the two data signals provides an ambiguous target bearing line. To resolve the ambiguity and refine the "fix" further "plants" are laid.
This method of submarine localization has a number of drawbacks which considerably reduce its efficiency particularly when dealing with faster and more manoeuverable modern submarines. For instance the time that an investigating aircraft requires to complete a sufficiently refined fix typically averages between 25 to 40 minutes and because of launching errors and uncertainties as to the exact trajectory of each sonobuoy, bearing errors can be as high as .+-.20 degrees.
Furthermore, this method is very costly as it often requires more than a dozen sonobuoys before a reasonably accurate fix can be defined.
The object of this invention is therefore to improve the localization capabilities of antisubmarine warfare equipment. More specifically an object of this invention is to provide a sonobuoy system capable of rapidly detecting the signals from and to a distant submarine using only the information received from a single buoy. To this effect the sonobuoy must carry at least three hydrophones to provide an unambiguous bearing, and the signal to ambient noise ratio of the acoustic information arriving at the hydrophone array must be sufficiently high to allow a usable bearing accuracy to be obtained.
A first proposal suggests the use of a station of three hydrophones tied to one another by means of flexible cables or ropes. However, due to the relative motion of the hydrophones the geometry of the array would not be sufficiently stable for rapid bearing determination and difficulties would be encountered in properly laying and maintaining the array.
A second proposal involves the use of a rigid structure for supporting the hydrophones. However, it is well known that underwater rigid structures generate considerable self noise, due to turbulence flow, resonance, etc., which would be immediately picked up by sensitive hydrophones and would render the data signals practically useless for beam steering processing.